This reverts an earlier change 4aeec2da

Instead of returning 0-filled buffers after EOS from AudioTrack we do this
work in AudioOutput instead. That way the EOS signal (0 frames returned)
is preserved in AudioCache which otherwise would lead to a heap size overflow
filling everything with zeroes.

Change-Id: I7e07429ba887957a4340dd4b21eef4bba76248cd
related-to-bug: 3514073